Connecting pieces interconnect the carburetor and the combustion chamber. The fuel-air mixture generated in the carburetor passes through the connecting piece to reach the combustion chamber of the engine.
Known connecting pieces of this kind are configured as smooth as possible on their inside wall to keep flow resistance between the carburetor and the combustion chamber, and the attendant power losses, as low as possible.
During operation, when the fuel-air mixture flows from the carburetor to the combustion chamber, some of the fuel is deposited on the inner wall of the connecting piece. When the inner wall is smooth, a closed film of fuel forms here which is particularly disadvantageous in the lower rpm range (idling range). The fuel film deposited on the inner wall, which has a considerably greater viscosity than the fuel that is dissolved in droplet form, flows inside the connecting piece along the connecting piece wall back to the carburetor or into the combustion chamber, depending on the pressure conditions. More precise testing has shown that the fuel film initially flows back toward the carburetor, where at the interface, between carburetor and connecting piece, for instance, at an edge between the connecting piece and the carburetor, it backs up until it is abruptly aspirated virtually in its entirety into the combustion chamber. This abruptly delivered quantity of fuel results in a pronounced overenrichment of the mixture, causing the engine rpm to drop suddenly, until a normal mixture is again delivered for combustion.
This aspiration into the combustion chamber of the fuel film formed on the inner wall of the connecting piece takes place periodically. In a two-stroke engine of a motor-driven chain saw, this process is repeated at approximately ten-second intervals. During the time when the fuel film is building up on the connecting piece wall and backing up there, fuel is taken from the fuel-air mixture, so that an overly lean mixture is supplied to the engine. When the fuel film detaches from the wall, the mixture composition abruptly changes into an overly rich mixture. This causes considerable fluctuations in the idling rpm, which may be in the order of magnitude of 500 rpm, for example. These idling rpm fluctuations occur particularly in modern, lightweight engines, in which the revolving masses have been largely reduced, so that angular momentum provides only limited speed compensation. Without structural changes, the problem can be solved only by setting the idling rpm correspondingly higher, which is uneconomical and also involves the risk that the clutch engagement rpm will be exceeded in tools having centrifugal coupling. Exact setting of the idling rpm is impossible because of the fluctuations that occur and these fluctuations also unnecessarily increase engine wear.
In tools that are operated not merely in one particular position but also in unusual positions, having for instance to be rotatingly manipulated through 180.degree., these rpm fluctuations during idling occur to an increased extent when the tool is swung about, because with the abruptly changing pressure conditions in the carburetor and the influence of gravity, the fuel film often migrates along the connecting piece wall from the carburetor to the combustion chamber. This triggers speed fluctuations that can cause misfiring and even stalling of the engine, especially when there is a sudden, severe overenrichment of the mixture.
A connecting piece having a textured inner wall, which is intended to lessen the sudden detachment of the film from the wall, is already known. This connecting piece has annular protrusions of rectangular cross section disposed one behind the other in axial direction. The protrusions are interrupted by channels extending in the axial direction of the connecting piece so that a plurality of rows of rectangular protrusions are formed and distributed over the periphery.
This connecting piece still does not solve the problems addressed above because the relatively large protrusions on the inner wall can not retain relatively large portions of the fuel film deposited thereon when aspiration into the combustion chamber takes place. In particular, the axially parallel longitudinal channels between the protrusions cause relatively large quantities of fuel to reach the combustion chamber abruptly, for instance, when the tool is swung about, and because of the altered pressure conditions in the carburetor, liquid fuel reaches the wall of the connecting piece. This fuel then runs along the axially parallel channels directly into the combustion chamber and causes a brief drop in rpm. Accordingly, even this connecting piece having structure on the inner wall does not provide a satisfactory solution.